1. Field of the Invention
The present invention relates to an image reader well suited for reading bound documents such as books and magazines.
2. Description of the Related Art
Image readers have been commercialized as book scanners. In such devices, an image sensing unit is arranged above a document table so as to have an open space of several tens of centimeters between said image sensing unit and said document table. When reading a bound document (hereinafter referred to as "book document") such as a book, magazine and the like, a user places the book document on the document table in an open state facing upward. Scanning of the document surface (reading target surface) begins when the start key is turned ON, and the document image read in conjunction with said scan is sequentially output to an external device. If the external device is a printer, a copy image is formed in real time. When using a book scanner, the operation of reading multiple pages is accomplished with high efficiency because the pages can be turned while the document is on the document table. Furthermore, damage to the book is markedly reduced in comparison to arrangements wherein the book is placed in an open state facing downward. The aforesaid book documents include file-type documents. Unlike single sheet documents, the document surface of book documents are curved. Therefore, distortion corresponding to said state of curvature appears in the read image. A read image corrected for said distortion is output to the external device, i.e., a read image is output which has an image quality identical to the read image of a sheet document. Methods of detecting the state of curvature of a document surface include a method of image analysis accomplished by reading the so-called "head" portion of a book document (Japanese Laid-Open Patent Application No. 5-161002).
In reading book documents, the edge (the so-called "fore-edge" of the book) of overlapping sheets appearing at bilateral sides of the document surface when a book document is in an open state are read together with the document surface. The read image of this fore-edge is unnecessary information and detracts from the appearance of the output image. Conventionally, there are two well-known methods of outputting an image which does not include the read image of the fore-edge. A first method utilizes the difference in luminance between the edge and the background of the document surface to detect the border between the edge portion and the document surface in the read image so as to erase the image outside said border. A second method detects the inflection point of the change in height of the document surface, and erases the read image on the exterior side of said inflection point.
However, in the first method utilizing the difference in luminance, disadvantages arise inasmuch as the document surface and edge portion cannot be distinguished when there the difference in luminance is slight as in the case, for example, of old books documents wherein the pages have become yellowed or faded. Furthermore, in the second method of checking for changes in the height of the document surface, disadvantages arise inasmuch as errors readily occur in the inflection point detection because a clear inflection point may not exist as in the case of relatively thin documents such as magazines and thin volumes.